Apparatus for heating liquids, and in particular for heating cooking oils, such as deep fat fryer-type apparatus, especially such apparatus as are used in commercial environments, such as fast food restaurants, must be able to function under conditions and circumstances which are not encountered in residential environment circumstances.
For example, in a typical residential cooking situation, a pot, kettle or frying pan is filled with, at most, a pint or so of oil, used for at most a few loads of food to be cooked, and then the oil is discarded, or "recycled" in the kitchen for a lesser use. In a commercial environment, however, several gallons of increasingly more expensive cooking oil are placed in a fryer at one time, are then used for repeated loads of food to be cooked, and the oil typically remains in the fryer for as long as the oil can be made to last.
The oil, of course, is in many ways a fragile ingredient. Indeed, as with extended or excessive heating, the oil may begin to chemically break down, and become unacceptable for cooking. In addition, the oil is exposed to thermal cycling, as heat is applied, and removed either through the application of the heat, or through absorption of the heat by the relatively cold food articles.
The operation of such a fryer must attempt to meet several competing goals. It is desired to maintain the oil at a sufficiently hot temperature to minimize absorption of the oil in to the food items, but at the same time not be wasteful of heating energy. Furthermore, the oil must not get so hot that chemical breakdown or fire becomes a concern.
An additional goal of such fryer apparatus is to apply the heat in as uniform manner as possible, so that there are no "hot" or "cold" regions in the fryer, which may affect the taste and quality of the food items, as well as representing wasted heating energy.
Electrically-heated fryers, of course, are capable of fine control of the rate and amount of heat which may be put into the cooking oil. However, electrical resistance heating of cooking oil can be expensive in terms of energy cost. Gas-fired fryers, which can be more efficiently operated, have been known to be difficult to operate and control. In addition, heating of oil by direct heating of a vessel by flame, creates thermal stresses in both the vessel and the burners, leading to shortened useful life, and increased operating costs.
One known method for heating a liquid such as a cooking oil, which comprises such an attempt to meet the several competing goals previously mentioned, is to pass a heat transfer fluid in a heat transfer relationship, past or through a cooking oil. The heat transfer fluid is first heated at a position thermally, if not physically, remote from the cooking oil.
For example, in Dunham, U.S. Pat. No. 4,905,664, a gas-fired grill is provided with a series of tubes, through which the hot combustion product gases are passed. The heated tubes may be used as a grilling surface, or the tubes may be immersed, so as to provide a heating source for cooking oil.
One potential drawback of the Dunham '664 patent, however, is that the amount and rate of heat which the combustion gases can transfer is relatively limited. If a substantial load, such as a large quantity of frozen food, is placed in the cooking oil, whatever heat is in the oil may be quickly absorbed, and the fan-drawn combustion gases simply may not be able to quickly replenish the heat so as to maintain the oil at an acceptable temperature, resulting in thermal cycling of the oil and potential risk of absorption. Even if the gas supply were substantially stepped up, fine control of the heat applied would be difficult, creating potential hot spots in the oil.
Another prior art apparatus is disclosed in Lazaridis et al., U.S. Pats. Nos. 3,985,120 and 4,091,801. In these references a fryer construction comprising an open-topped cooking vessel which has a bottom and/or side portion which shares a common wall with a sealed chamber in which a fluid is contained, is disclosed. The sealed chamber ideally contains only the liquid and/or gaseous phases of a vaporizable heat transfer fluid, with no other non-condensable material in the chamber. The principle of operation of this apparatus is that the heat transfer fluid is heated to vapor in a lower or removed portion of the sealed chamber, and condenses against the cooler wall which is shared by the cooking vessel. In this manner, heat is transferred to the cooking oil, so long as the cooking oil is at a heat and temperature lower than the heat of condensation of the heat transfer fluid vapor, in theory preventing overheating of the cooking oil.
However, the apparatus disclosed in the Lazaridis et al. patents require the maintenance of a specially constructed sealed chamber, which increases manufacturing costs. In addition, the very process which is used to prevent overheating of the cooking oil limits the available rate at which heat can be delivered to the oil, which in turn limits the rate at which the cooking oil can recover from a large load, such as a large quantity of frozen food being lowered into the cooking oil. The cooking oil can only be heated as fast as the transfer fluid can be boiled, transported by convection to the common wall, lose its heat through the common wall, condense, and be reheated. Even if the transfer fluid is continuously heated, which would be wasteful, so as to establish a more-or-less steady-state situation, as mentioned previously, a vapor-to-liquid heat transfer is a less efficient process than, for example, a liquid-to-liquid transfer process. Still further, the usage of a common heating wall, results in the cooking oil becoming heated from the common wall outward (or upward), thereby resulting in an unavoidable temperature gradient from bottom to top or from side to side, of the volume of cooking oil.
It is, accordingly, desirable to provide a method and apparatus for indirectly heating a volume of cooking oil, through indirect means, such as a heat transfer fluid heated at a position thermally and/or physically remote from the cooking oil, which is capable of close control of the rate of heat transfer into the cooking oil.
It is additionally desirable to provide such a method and apparatus which is capable of supplying, on demand and at an enhanced rate, large quantities of heat to the cooking oil, such as upon substantially loading of the cooking oil with large amounts of cold or frozen food to be cooked.
It is further desirable to provide such a method and apparatus in which the supply of heat to the cooking oil may be quickly curtailed, such as when the load on the cooking oil is removed, such as by removal of the cooked food, which may still be substantially cooler than the cooking oil.